Multi-engine aircraft, such as passenger jets, operate in multiple modes during any given flight, with the majority of the flight spent in a cruise mode. Because of uneven wear and/or uneven age of the aircraft engines, the actual thrust output of each of the engines can vary even when conventional thrust metrics such as Engine Pressure Ratio (EPR) and Low Rotor Speed (N1) indicate approximately identical thrust outputs.
During cruise mode, if the multiple engines are outputting uneven (non-symmetrical) thrust, aircraft control surfaces are used to maintain a correct aircraft heading, and ensure that the aircraft does not go off course. Continuously operating the aircraft with asymmetric thrust and aircraft control surface corrections results in a decrease in fuel efficiency of the aircraft and an increase in the fuel costs for any given flight.